Meteor-ablated Aluminum in the Mesosphere-Lower Thermosphere

The first global atmospheric model (WACCM-Al) of meteor-ablated aluminum was constructed from three components: The Whole Atmospheric Community Climate Model (WACCM6); a meteoric input function for Al derived by coupling an astronomical model of dust sources in the solar system with a chemical meteoric ablation model; and a comprehensive set of neutral, ion-molecule and photochemical reactions relevant to the chemistry of Al in the upper atmosphere. The reaction kinetics of two important reactions that control the rate at which Al+ ions are neutralized were first studied using a fast flow tube with pulsed laser ablation of an Al target, yielding k(AlO+ + CO) = (3.7 ± 1.1) × 10−10 and k(AlO+ + O) = (1.7 ± 0.7) × 10−10 cm3 molecule−1 s−1 at 294 K. The first attempt to observe AlO by lidar was made by probing the bandhead of the B2Σ+(v′ = 0) ← X2Σ+(v″ = 0) transition at λair = 484.23 nm. An upper limit for AlO of 60 cm−3 was determined, which is consistent with a night-time concentration of ∼5 cm−3 estimated from the decay of AlO following rocket-borne grenade releases. WACCM-Al predicts the following: AlO, AlOH and Al+ are the three major species above 80 km; the AlO layer at mid-latitudes peaks at 89 km with a half-width of ∼5 km, and a peak density which increases from a night-time minimum of ∼10 cm−3 to a daytime maximum of ∼60 cm−3; and that the best opportunity for observing AlO is at high latitudes during equinoctial twilight.